Secondary electron collection system



Oct. 14, 1969 A. B. EL-KAREH ET AL 3, 7

SECONDARY ELECTRON COLLECTION SYSTEM Filed Aug. 26, 1966 ELECTRON BEAM SOURCE VIDEO AMPLIFIER DEFLECTION CIRCUIT I(ground) I(collecfor) I(c0i|)= 03A 0.5A 0.75A LOA INVENTORS AUGUSTE 8. EL- KAREH o l I CHARLES R. MARSH o 5' 1 200 BY RICHARD B. FAIR F g. 5 ENERGY (ev) I 1&2 H S E ATTORNEY United States Patent 3,472,997 SECONDARY ELECTRON COLLECTION SYSTEM Auguste B. El-Kareh, Potsdam, N.Y., Charles R. Marsh, Boalsburg, Pa., and Richard B. Fair, Durham, N.C., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Aug. 26, 1966, Ser. No. 576,179 Int. Cl. H05b .7/18

U.S.Cl. 219-121 6 Claims ABSTRACT OF THE DISCLOSURE A toroidal section supplied with DC current is utilized to provide a curved magnetic field which directs and carries away from a workpiece secondary electrons liberated therefrom upon the electron bombardment thereof. The toroidal section channels the secondary electrons to a shielded scintillator which is connected to a photomultiplier tube, the output of which supplies an image forming CRT to display thereon a visual replica of the topography of the workpiece.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates generally to improvements in electron machine-scan systems and the like and more particularly to a new and improved secondary electron collection and imaging system for continuously observing the progress of a machining operation on a workpiece.

In order to meet the increased demand for reliability standards in microelement processing, it is necessary to observe carefully the specimen being manufactured before, during and after electron bombardment. Many electron beam machines are equipped with an optical microscope. However, the resolving power of any optical system with such a large focal length is very poor. Further, a prism mounted inside the electron beam column will'become coated with vaporized material from the workpiece, and be rendered ineffective.

Techniques for utilizing electron image formation within the electron beam machine have also been employed. These techniques employ a single electron beam'suitably biased for either machining or image formation. In the image formation mode the intensity of the electron beam is reduced sufficiently to prevent excessive heating of the workpiece, while still causing secondary electron emission from the workpiece. Such a system is described in US. Patent 3,196,246 by one of the present inventors. As described therein, during the image formation or scanning mode, secondary electrons are removed from the workpiece by employing a collector electrode positioned adjacent the workpiece. The secondary electrons attracted thereto are then passed through a resistor which by virtue of the increased current flow therethrough produces signals which are detected, amplified and applied to a display device such as an oscilloscope upon which a picture or image of the workpiece is produced.

The general purpose of the present invention is to provide an improved apparatus for collecting secondary electrons liberated from an electron bombardment of the workpiece and conveying the electrons emitted thereby to an image forming device which is out of the line-0fsight of the workpiece and shielded from any light shining on the workpiece due to the filament in the electron gun. In a preferred embodiment of the present invention, a curved magnetic field is employed to carry the collected secondary electrons away from the workpiece to a shielded scintillator and photomultiplier tube and then to an image forming device such as a cathode ray tube.

3,472,997 Patented Oct. 14, 1969 "ice An object of the present invention is to provide an improved apparatus for collecting secondary electrons from a specimen for image formation that substantially eliminates the aforementioned prior art disadvantages and provides continuous monitoring of the specimen during an entire electron beam machining operation.

Another object of the invention is to provide, in combination with an electron beam apparatus, a novel link in an imaging system wherein image quality and contrast are improved while providing effective shielding of the imaging apparatus from both the ambient light and the vaporized material produced by the electron beam and thereby substantially reduce the unwanted noise generated by the electro multiplier.

Still another object is to provide a toroidal segment wherein secondary electrons are collected and spiraled through the toroidal core to the imaging apparatus for displaying electron micrographs.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic view of an electron machinescan device and the toroidal segment for collecting and transferring secondary electrons from the workpiece in accordance with a preferred embodiment of the present invention;

FIG. 2 shows an enlarged partial front view of the toroidal segment of FIG. 1 looking into the toroidal segment; and

FIG. 3 is a plot of energy pass-band curves of a typical toroidal segment for various exitation currents.

Referring now to FIG. 1 of the drawing, there is shown an electron beam source 10 for providing a source of electrons. The source may contain, for example, an electron gun of the conventional tungsten hairpin filament typewith an accompanying Wehnelt cylinder or cup. A typical electron source which would be suitable for use herein is described in US. Patent No. 3,196,246. The main reasons for selecting this type gun are its simplicity of construction and the fact that any gun geometry, within wide limits, provides a brightness of the electron source suflicient to enable machine-scan operation as will be described hereinafter. A beam of electrons 12 accordingly is emitted from the source 10 and passed through an anode 11 for accelerating the electrons to a desired velocity. A focusing coil 14 is disposed along the path of the electron beam for focusing the beam. While the focusing coil has been illustrated as a single element, it is to be understood that a plurality of lenses could be employed for focusing the electron beam without departing from the spirit of the present invention. Further, a condensing lens just beyond the electron source and an objective lens near the workpiece could be employed for accomplishing demagnification of the beam as described in the above-referenced patent.

Dis-posed between the lens 14 and a work plane 16 is a deflection network 18 for scanning the electron beam across a workpiece 20 in a prescribed manner during the scanning .mode of machine operation. For example, the deflection network 18 may be either magnetic or electrostatic with both horizontal and vertical deflection coils or plates in accordance with the particular design requirements. A typical magnetic deflection system is illustrated in the above-referenced patent.

The deflection network 18 is driven in synchronism with the deflection plates or yokes of a cathode ray tube 22 by a deflection circuit 24 which may comprise the usual sawtooth generators for accomplishing deflection in both the horizontal and vertical planes. For example, the primary electron beam 12 may be scanned across the workpiece 20 in the horizontal plane at a rate of 525 times per vertical scan as in a standard television system.

As a result of scanning the workpiece 20, by the primary beam 12, both reflected electrons and secondary emission electrons emanate from the workpiece or specimen 20. However, for reasons to be discussed hereinafter, the invention herein is concerned mainly with the secondary electron emission. It has been found that as the primary electron beam scans a workpiece, the secondary electrons liberated therefrom will vary in accordance with the topography of the specimen. This variation in secondary emission is primarily due to the dependence of the number of secondary electrons liberated, on the angle of incidence of the primary beam. The energies of these liberated electrons have been found to range up to 150 electron volts. Accordingly, this variation in secondary emission can be used to cause a similar variation in contrast in an electronic image displayed on the CRT 22 as will be described hereinafter.

Another method of producing contrast is to use reflected electrons; however, these electrons tend to be emitted in a plane normal to the surface and at an angle opposite to the incidence angle of the primary beam. Further, since these electrons have such high velocities, it is impossible to observe any part of the target which is not in a direct-line path with the collector. While higher contrast images can be achieved with the use of reflected electrons, more detailed pictures can be obtained 'by using secondary electrons. Additionally, since the collector must be in a direct line with the target to make use of the reflected electrons, all surfaces in the vicinity of the workpiece would become coated with vaporized material during the machining operation. Thus, a scintillator or a first dynode of an electron multiplier placed near the workpiece would be rendered useless in a very short time. Therefore, the electron detector of the machinescan device must be out of the line-of-sight of the specimen and must be shielded from any light shining on the specimen due to the filament. Thus, slow secondary electrons are preferable to reflected electrons in the image formation process.

Rather than employ a collector electrode as described in the above-referenced patent, it is desirable to collect the secondary electrons and carry them away from the workpiece to an image forming device remote from the work plane. In accordance with a preferred embodiment of the invention, a curved magnetic field may be employed to accomplish this collecting and conveying operation. In particular, FIG. 1 illustrates a toroidal segment having a curved metallic tube 26 and a coil of wire 28 wrapped along the length thereof. By applying a direct current through the coil a magnetic field along the axis of the tube will be generated and cause the collected electrons to spiral through the tube 26.

The electrons gain access to the toroidal core segment through an input collector generally referred to in FIG. 1 by the numeral 30. A frontal view of the input to the toroidal core segment is illustrated in FIG. 2 as having a vapor shield 32 around the periphery of the tube inlet and a screen mesh or grid 34 biased by a high voltage supply for attracting the secondary electrons emitted from the workpiece. The grid 34 is insulated from the shield 32 and the tube 26 by insulation material 35. The vapor shield 32 is connected to a magnetic shield 33 which prevents the magnetic field induced in the winding 28 from causing undesirable primary beam deflections. It is to be understood, however, that the vapor shield 32 could be insulated from the magnetic shield 33 and connected to a negative bias supply for shaping the electron collecting field created by the grid 34 for more efficient secondary emission collection.

Secondary electrons emitted from the workpiece will then enter the tube at the input collector section and as a result of the magnetic field created by current passing through the coil of wire, the electrons will spiral around and through the core to the exit end of the segment where an electric field created "by a high voltage scintillator 36 will accelerate the low energy secondary electrons and give them enough energy to excite the scintillator. The scintillator may, for example, be a NaI crystal coated with a 150 angstrom layer of aluminum to which the high voltage lead 37 is attached. The crystal is then optically connected to a Lucite light pipe 38 by a drop of vacuum oil placed between the two surfaces. The function of the light pipe is to convey the light signal from the scintillator to the photocathode of a photomultiplier tube 40 such as a Dumont tube, model 6292. The light pipe 38 may be connected to the photocathode in a similar manner as the crystal is connected to the light pipe. The scintillator, light pipe and photomultiplier are shielded from the ambient light by a metal shield 42.

The electronic signal produced by the photomultiplier is amplified in a video amplifier 44 having the requisite bandwidth for signals issuing from the photomultiplier. The output of the video amplifier is then applied to the intensity grid of the cathode ray tube 22 for modulating the scanning beam therein.

While the actual dimensions of the toroidal segment are not critical, in order to provide an effective imaging system, the shape of the segment should conform to certain design criteria. Specifically, the coil should produce an axial magnetic field of suflicient magnitude that will allow electrons of energies up to electron volts to pass through its core. Further, this magnetic field should be created with a minimum amount of excitation current in order to keep Joulean heat generation to a minimum. Additionally, the core should curve away from the workpiece in such a way so as to shield the scintillator at the exit end from evaporated material and light generated at the workpiece. A typical toroidal segment which meets these requirements is described below for purposes of illustration only and not by way of limitation. For example, the curved metallic tube 26 may be made of inch inside diameter copper tubing having an arc length of 5% inches on a radius of 3- /2 inches. The coil of wire 28 could be wound with 1600 turns of No. 22 enamel covered wire. The vapor shield and grid could be attached to one end of the tube and firmly attached to the work plane 16 and magnetic shield 33 as illustrated in FIG. 1.

FIG. 3 illustrates the energy pass'band characteristics for the above-described toroidal segment for several exciting currents. The horizontal axis is representative of the energy of the secondary electrons emitted from the workpiece and the vertical axis is representative of the ratio of current lost to the tube to the current passing through the tube. The data for plotting the curves was obtained by passing electrons into one end of the toroidal segment and collecting those which passed through the segment on a high voltage collector in order to measure the current that passed through the hollow core. Also, the current that passed to ground (the number of electrons that spiraled into the walls of the core) was measured. By varying the energies of the entering electrons, for various coil excitation currents, the curves of FIG. 3 were obtained. It can be readily seen from these curves that for relatively low exciting currents, sufficient magnetic fields can be created inside the core for passing the range of electron energies necessary in image formation. It should also be noted that the energy pass-band curves are for a combination of paraxial and nonparaxial electrons. Therefore, the curves of FIG. 3 are valid for the average electron in a spectrum of electrons entering the core at various points off the core axis.

In operation then, the electron bear 12 is accelerated to the workpiece 20 by a voltage appearing on the anode 11. The magnetic deflection coils or deflection plates 18 deflect the electron beam along a preset path in accordance with the desired welding, etching, drilling or cleaning operation. When the electron beam is not operating in the machining mode, the deflection circuit 24 causes the deflection network 1 8 to deflect the electron beam in synchronism with the deflection network on the cathode ray tube 22. The secondary electrons emitted from the workpiece during this scanning operation are collected and conveyed through the toroidal segment in a manner as described previously, to the scintillator 36, through the light pipe 38 and thence to the photomultiplier tube 40. The electrical signal issuing therefrom is amplified in the video amplifier 44 and applied to the intensity grid of the cathode ray tube 28. In this way, as the electron beam scans the workpiece, the signal created by collecting the secondary electrons from the workpiece causes the formation of an image or micrograph of the workpiece on the face of the cathode ray tube. By using this technique, it is possible then, not only to machine and observe the machining operation continuously, but it is also possible to inspect a final work product.

The invention described therein accordingly provides an improved apparatus for collecting and conveying secondary electrons from a specimen to an image forming system wherein unwanted noise signals generated by the primary electron beam are substantially reduced.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In combination with an electron beam apparatus wherein an electron beam is directed onto a workpiece for performing a machine-scan operation thereon, a device comprising:

a curved toroidal segment for magnetically conveying emitted secondary electrons away from said workpiece including a tubular core and a coil of wire wound on said core for providing a like curved magnetic field within said core, whereby secondary electrons entering said core are spiraled around and through said core to the exit end thereof; and

means receiving the conveyed electrons for providing an electrical signal as a function of said conveyed electrons.

2. The device as recited in claim 1 wherein said means receiving the conveyed electrons comprises:

a chamber;

a scintillator within said chamber for emitting light in response to said conveyed electrons; and

a photomultiplier responsive to said light for providing said electrical signal as a function of the intensity of said light.

3. The device as recited in claim 2 further comprising:

means intensity modulated by said electrical signal for providing a visual display of said workpiece.

4. The device as recited in claim 3 further comprising:

a deflection system for deflecting said electron beam in synchronism with said means for providing a visual display, whereby an image of said workpiece is formed on said visual display.

5. An apparatus as recited in claim 1 wherein:

a circular vapor shield is mounted at the inlet end of said core; and

a grid within said shield and insulated therefrom for attracting said electrons.

6. An apparatus as recited in claim 1 further comprismg:

a magnetic shield interposed between said toroidal segment and said workpiece for preventing the magnetic field from said segment from deflecting said electron beam.

References Cited UNITED STATES PATENTS 2,769,911 11/1956 Warmoltz 250-41.9 2,928,943 3/1960 Bartz et a1 250495 3,056,027 9/ 1962 Martinelli 250-41.9 3,146,347 8/1964 Ziegler 25o 49.5 3,158,733 11/1964 Sibley 219121 3,196,246 7/1965 El-Kareh 219-121 3,221,133 11/196-5 Kazato et a1. 219121 3,223,837 12/1965 Shapiro et al 250495 3,351,755 11/1967 Hasler 25049.5 3,381,132 4/1968 Okano 25049.5

OTHER REFERENCES Wide Band Detector for Micro-Microampere Low- Energy Electron Currents, by T. E. Everhart et al., Journal of Scientific Instruments, vol. 37, July 1960, pp. 246- 248.

JOSEPH V. TRUHE, Primary Examiner W. DEXTER BROOKS, Assistant Examiner 

